CN101666672B - Micro-mass detection sensing circuit based on FBAR - Google Patents

Abstract

The invention discloses a micro-mass detection sensing circuit based on FBAR, comprising a first FBAR, a second FBAR, a first N frequency dividing circuit, a second N frequency dividing circuit, a timer, a first counter, a second counter, a subtractor, a sensor information processing module and a clock source, wherein, the first FBAR is used as a sensing source and the second FBAR is used as a reference source; the first N frequency dividing circuit and the second N frequency dividing circuit respectively carry out frequency conversion on signals output by the first FBAR and the second FBAR; the timer generates a periodic counting signal based on a clock source; triggered by a counting signal, the first counter and the second counter respectively count the signals output by the first N frequency dividing circuit and the second N frequency dividing circuit; the subtractor receives the counting results of the first counter and the second counter and calculates the difference value of the counting results; the sensor information processing module receives the results calculated by the subtractor and calculates the mass loaded by the micro mass; and the clock source drives the first counter, the second counter, the timer and the sensor information processing module to work synchronously. A detection system of the invention detects the mass of microscopic substances through the two FBARs, and has high sensitivity and accurate detection.

Description

A kind of little quality testing sensing circuit based on FBAR

Technical field

The present invention relates to little quality testing field, relate in particular to a kind of little quality testing sensing circuit based on FBAR.

Background technology

Piezoelectric sensor has wide practical use in little quality testing field, its sensing principle is that piezoelectric sensor is operated in resonant condition, when mass loading deposits to the vibrating mass surface, its resonance frequency can be drifted about, detect the drift value of frequency, by calculating the quality size that just can obtain deposited material, its sensing responsive degree depends primarily on frequency of operation and quality factor q again.

This class sensor mainly contains surface acoustic wave sensor (SAW) and acoustic wave sensor (BAW) at present.SAW utilizes interdigital electrode to transmit and receive surface acoustic wave to come sensing, and advantage is simple in structure, is convenient to signal transformation and processing, and shortcoming is that volume is bigger, and incompatible with semiconductor technology.BAW utilizes the bulk acoustic wave of compressional wave in the piezoelectric crystal or shear mode to come sensing, at present the little balance of quartz wafer (QCM) of existing comparative maturity.

Thin film bulk acoustic resonator (FBAR) also is a kind of of BAW, the sandwich structure that its structure is made up of piezoelectric membrane and its upper and lower surface electrode of half-wavelength, the upper/lower electrode effect is feed-in electric field and formation sound total reflection layer, and piezoelectric membrane (mainly being AlN or ZnO at present) produces the compressional wave resonance of radio band sound wave under this electric field.

FBAR has the unexistent advantage of other sensor as sensor: 1. FABR frequency of operation height can reach that 20GHz is above, quality factor is high, Insertion Loss is low, has high susceptibility, can realize the sensing of nanogram magnitude.This is that susceptibility is the highest in the moment sensor.The Hao Zhang of University of California in 2005 etc. has studied the FABR sensor of Al/ZnO/Al and air chamber structure, and the susceptibility of its mass sensitivity is 57 times of QCM.2. low-power consumption and good radio transmission characteristics are the desirable node solutions of radio sensing network (WSN).3. the compatible semiconductor technology of FBAR can be integrated in sensing and wireless receiving and dispatching among the IC, and can the microcell sensing.4. simple in structure, dependable performance, with low cost.

Summary of the invention

The invention provides a kind of highly sensitive little quality testing sensing circuit based on FBAR.

A kind of little quality testing sensing circuit based on FBAR comprises:

Be used to detect little mass loading as a FBAR of sensing sources with as the 2nd FBAR with reference to the source;

The one FBAR output signal is carried out first Fractional-N frequency circuit of frequency conversion and the second Fractional-N frequency circuit that the 2nd FBAR output signal is carried out frequency conversion;

Produce the timer of periodic counting signal based on the clock source;

Trigger first gauge that the first Fractional-N frequency circuit output signal is counted and second counter that the second Fractional-N frequency circuit output signal is counted by count signal;

Receive first counter and the second rolling counters forward result, calculate the subtracter of both differences;

Receive subtracter result of calculation and calculate the heat transfer agent processing module of the quality of little mass loading;

Drive the clock source of first counter, second counter, timer and heat transfer agent processing module synchronous working.

The top electrode of a described FBAR or lower electrode surface are deposited with small material adsorbent thin film, can adsorb one or more specific small materials, the small material that specific thin film adsorbs is specific, for example deposit TiO ₂, there is OH freely on its surface ^-And Ti-O ^-, can adsorb K ⁺, deposit Au, adsorbable Hg ²⁺But, organism such as deposition film adsorbed proteins, DNA also.

Described the 2nd FBAR top is the small material adsorbent thin film of deposit not, and perhaps sealing does not contact with the external world, makes it insensitive to the small qualitative factor in the working environment.

A described FBAR is identical with the 2nd FBAR original resonance frequency, and is in the identical working environment, can difference offset influences such as neighbourhood noise, temperature drift.

The described first Fractional-N frequency circuit and the second Fractional-N frequency circuit are reduced to the manageable frequency of circuit with the resonance frequency of FBAR, be to realize frequency sampling, and the frequency of reducing to is usually less than 1/2 of the frequency of operation in clock source; Realize that for convenient frequency dividing ratio N gets 2 integral number power usually.Frequency dividing ratio N is more little, and the precision of whole little quality testing circuit is high more, preferably is arranged on 2 ⁶～2 ¹⁰

In the range of sensitivity of sensor, the counting trigger pip cycle that described timer produces is long more, and the precision of whole little quality testing circuit is high more, and promptly circuit can detect little mass loading of littler quality, and its principle of work is as follows:

The original resonance frequency of the one FBAR is f _s, adhere to little mass loading after, its resonance frequency becomes f _s', it is Δ f that resonance frequency changes size, therefore has:

f _s′＝f _s-Δf _s (1)

And as the 2nd FBAR with reference to the source, resonance frequency still remains f _s

Behind Fractional-N frequency circuit Fractional-N frequency, the sensing sources signal frequency split becomes

The reference source signal frequency division becomes

Timer can produce periodic count signal, in a count cycle T, the first Fractional-N frequency circuit and the second Fractional-N frequency circuit output signal is counted, and the sensing sources pulse number is The reference source pulse number is

Reference source count pulse number deducts the sensing sources count pulse and gets both step-by-step counting difference S, and is as follows:

$S=\frac{1}{N}\mathrm{T\Δ}{f}_s---\left(2\right)$

S is an integer generally speaking.

When S 〉=1, can reach a conclusion: detected little mass loading attached on the FBAR.So the change of resonance frequency Δ f that system can detected minimum mass causes will make S=1 at least, this is the condition that the accuracy of detection of system must satisfy.Have this moment:

$\frac{1}{N}\mathrm{T\Δ}{f}_s=1---\left(3\right)$

Described heat transfer agent processing module draws the quality that is adsorbed on the little mass loading on the FBAR by the quality of little mass loading and the corresponding relation of frequency-splitting, and its principle is as follows:

Fig. 4 (a) and Fig. 4 (b) are non-loaded and BVD (Butterworth-Van Dyke) equivalent electrical circuit of a FBAR under the loading condition, C in this equivalence circuit arranged for testing circuit of the present invention ₀Be the direct capacitance of FBAR, C _m, L _m, R _mDynamic capacity, dynamic inductance and the return loss of representing a FBAR respectively, they are all relevant with the load of the physical parameter of FBAR and increase.

$C_0=\frac{\mathrm{\ϵA}}{d_0}---\left(4a\right)$

$C_m=\frac{{8k}_t^2{C}_0}{{\mathrm{\π}}^2}---\left(4b\right)$

$L_m=\frac{1}{4{\mathrm{\π}}^2{f}_S^2{C}_m}---\left(4c\right)$

$R_m=\frac{{\mathrm{\η}}_0}{{\mathrm{\ρ}}_0{v}_0^2{C}_m}{\left(\frac{f}{f_S}\right)}^2---\left(4d\right)$

${\mathrm{<figure-callout\; id="3"\; label="L"\; filenames="H2009101526085E00022.png"\; state="\{\{state\}\}">L</figure-callout>}}_3=\frac{4f_S{L}_m{\mathrm{\&rho;}}_3{\mathrm{<figure-callout\; id="3"\; label="d"\; filenames="H2009101526085E00022.png"\; state="\{\{state\}\}">d</figure-callout>}}_3}{{\mathrm{\&rho;}}_0{v}_0}---\left(4e\right)$

Wherein, ε is the piezoelectric layer conductivity, and A is the area in FBAR piezoelectric working district, d ₀Be piezoelectric layer thickness, k _t ²Be electromechanical coupling factor, η ₀Be the acoustics viscosity of piezoelectric layer, ρ ₀Be the density of piezoelectric layer, ρ ₃Be the density of little mass loading, d ₃Be the thickness of little mass loading of being increased, L ₃Be the variation that little mass loading is attached to the dynamic inductance that FBAR causes, v ₀Be the longitudinal wave velocity of piezoelectric layer, f is the stimulating frequency of FBAR, f _sBe the initial resonance frequency of FBAR and

$f_s=\frac{v_0}{2d_0}---\left(5\right)$

This equivalence circuit is operated in resonance frequency f at ordinary times _sTuning-points, when FBAR adsorbs little mass loading, can get its resonance frequency according to equivalent electrical circuit resonance and become f _s':

$f_S^{\&prime;}=\frac{1}{2\mathrm{\&pi;}\sqrt{{{L_m}^{\&prime;}C}_m}}---\left(6\right)$

Wherein, L _m'=L _m+ L ₃

Can shift out formula (7) onto by above-mentioned equation

$\frac{\mathrm{\&Delta;}{f}_s}{f_s}=\frac{f_s^{\&prime;}-f_s}{f_s}=\sqrt{\frac{L_m}{{L_m}^{\&prime;}}}-1={(1-\frac{L_3}{L_m+L_3})}^{\frac{1}{2}}-1---\left(7\right)$

Because little mass loading is attached to the variable quantity L of the dynamic inductance that a FBAR causes ₃With respect to its dynamic inductance L _mVery little, i.e. L ₃＜＜L _m, can ask limit abbreviation (7) to get

$\frac{\mathrm{\&Delta;}{f}_s}{f_s}\&ap;-\frac{1}{2}\&CenterDot;\frac{L_3}{L_m+{\mathrm{<figure-callout\; id="3"\; label="L"\; filenames="H2009101526085E00022.png"\; state="\{\{state\}\}">L</figure-callout>}}_3}\&ap;-\frac{{\mathrm{<figure-callout\; id="3"\; label="L"\; filenames="H2009101526085E00022.png"\; state="\{\{state\}\}">L</figure-callout>}}_3}{2L_m}---\left(8\right)$

4 (c), 4 (e) and (5) substitution simplified:

$\frac{{\mathrm{\&Delta;f}}_s}{f_s}\&ap;-\frac{{\mathrm{\&rho;}}_3{\mathrm{<figure-callout\; id="3"\; label="d"\; filenames="H2009101526085E00022.png"\; state="\{\{state\}\}">d</figure-callout>}}_3}{{\mathrm{\&rho;}}_0{d}_0}=-\frac{{\mathrm{\&rho;}}_3{d}_{3}A}{{\mathrm{\&rho;}}_0{d}_{0}A}=-\frac{\mathrm{\&Delta;m}}{M}---\left(9\right)$

Wherein, Am is the quality of little mass loading, and M is the quality in the piezoelectric working district of FBAR, and formula (9) is classical Sauerbrey equation form, and the resonance frequency difference and the resonator absorption quality of expression acoustic resonator are linear.Negative sign represents that quality increases, and the FBAR resonance frequency reduces.Be noted that: the ratio on equation both sides is just set up less than 2% up-to-date style (9), it is 2% of the biggest quality load quality that can not surpass FBAR piezoelectric working district, this is because when mass loading was excessive, the prerequisite hypothesis based on perturbation of Sauerbrey equation had just lost efficacy.

When S=1, push away by (3) and (9)

$\mathrm{\&Delta;m}\&ap;-\frac{M\&CenterDot;N}{f_s\&CenterDot;T}---\left(10\right)$

Can reach a conclusion, the detected minimum mass of system's energy is directly proportional with the mass M in FBAR piezoelectric working district, with the resonance frequency f of FBAR _sBe inversely proportional to, homologous ray frequency dividing ratio N is directly proportional, and the homologous ray count cycle, T was inversely proportional to.When the FBAR sensing sources was determined, the mass M in piezoelectric working district was certain, the resonance frequency f of FBAR _sNecessarily, improve the accuracy of detection of system, only need reduce the frequency dividing ratio N of system, or increase the system counts period T, need be optimized both according to circuit resource usually.

The QCM mass area ratio responsiveness that resonance frequency is 40MHz is in the nanogram magnitude, and resonance frequency can reach the pik magnitude in the FBAR of 1GHz responsiveness, and precision is 1000 times of qcm sensor.By adjusting circuit parameter N and T, can reach the least limit that FBAR can detect small quality in theory.

When S＞1, push away by (2) and (9):

$\mathrm{\&Delta;m}\&ap;-\frac{M\&CenterDot;S\&CenterDot;N}{f_s\&CenterDot;T}---\left(11\right)$

Thus formula as can be known, poor by the step-by-step counting behind reference source and the sensing sources signal frequency split, can push away the quality of little mass loading on the FBAR top layer adsorbent thin film.

The little quality testing sensing circuit that the present invention is based on FBAR has the following advantages:

(1) detection system adopts two FBAR to detect small quality, overcomes external environmental interference with differential mode, can improve accuracy in detection;

(2) can improve the detection sensitivity height by adjusting circuit parameter, can reach the least limit that FBAR detects small quality in theory;

(3) circuit structure is simple, and function-stable realizes that cost is lower.

Description of drawings

Fig. 1 is the structural representation of the present invention the one FBAR;

Fig. 2 is the structural representation of the present invention the 2nd FBAR;

Fig. 3 is the synoptic diagram after the present invention the one FBAR adsorbs little mass loading;

Fig. 4 is the module diagram of circuit of the present invention;

Fig. 5 is circuit FBAR of the present invention non-loaded (a), the equivalent circuit diagram when load (b) is arranged.

Embodiment

As shown in Figure 1, a kind of FBAR 100 with small quality adsorbent thin film mainly comprises substrate 105, acoustic reflection layer 106, top electrode 101, bottom electrode 103, piezoelectric layer 102, and little quality adsorbent thin film 104, piezoelectric layer material selection AlN wherein, piezoelectric working district area is 100um * 100um.

As shown in Figure 2, a kind ofly mainly do not comprise substrate 205, acoustic reflection layer 204, top electrode 201, bottom electrode 203 and piezoelectric layer 202 with the FBAR 200 of small quality adsorbent thin film, piezoelectric layer material selection AlN wherein, piezoelectric working district area is 100um * 100um.

As shown in Figure 3, little quality adsorbent thin film 104 of FBAR 100 top layers has been adsorbed the little mass loading 108 of one deck.

As shown in Figure 4, a kind of little quality testing sensing circuit based on FBAR comprises a FBAR100 and the 2nd FBAR200, and FBAR100 deposited on top one deck adsorbent thin film wherein is as sensing sources.The 2nd FBAR200, not deposit adsorbent thin film is as the frequency reference source.

The upper and lower thickness of electrode of FBAR is very little, can ignore the resonance frequency influence of FBAR, and the resonance frequency of FBAR is mainly determined by piezoelectric layer.The longitudinal wave velocity of AlN material is 10400m/s, and for a FBAR100 and the 2nd FBAR200 of resonance frequency 2GHz, its AlN piezoelectric layer thickness all is about 2.6um.The density of AlN material is 3260Kg/m ³, the piezoelectric working district quality that calculates a FBAR100 and the 2nd FBAR200 is 84.76ng.

The signal output part of the one FBAR100 and the 2nd FBAR200 is connected the first Fractional-N frequency circuit 3 and the second Fractional-N frequency circuit, 4, the first Fractional-N frequency circuit 3 and the second Fractional-N frequency circuit 4 respectively and respectively the output signal of a FBAR100 and the 2nd FBAR200 is carried out frequency conversion.

The resonance frequency of FBAR oscillator wants high with respect to the common treatable frequency of CMOS integrated circuit (or saying with respect to the counter maximum operation frequency), CMOS integrated circuit (counter) is difficult to handle so high frequency, need carry out Fractional-N frequency to resonance frequency, generally get N 2 ⁶～2 ¹⁰Between, get N=256 in the present embodiment.

The output terminal of the first Fractional-N frequency circuit 3 and the second Fractional-N frequency circuit 4 is connected first counter 5 and second counter 6 respectively, first counter 5 also is connected with clock source 9 simultaneously with second counter 6, as shown in Figure 3, this clock source 9 (being obtained by digital circuit or oscillatory circuit shaping) is as counting clock, and actuation counter is counted the signal sampling under the frequency division in the count cycle.

Timer 8 is driven by clock source 9, has the periodic counting signal of fixed cycle in order to generation, and this count signal is a square wave.When count signal was high level, first counter 5 and second counter 6 began counting, and when count signal transferred low level to, first counter 5 and second counter 6 finished counting, and the present embodiment design number trigger pip cycle is made as 256ms.

Count results be input to first counter 5 and subtracter 7 that second counter 6 is connected in, subtracter 7 calculates the difference of two count results, this difference is made as 100 in the present embodiment.

The output terminal of subtracter 7 is connected with heat transfer agent processing module 10 simultaneously, and when counting finished, clock source 9 drove heat transfer agent processing module 10 and calculates the quality of the little mass loading 108 that is adsorbed on a FBAR100 according to the difference of count results, for

$\mathrm{\&Delta;m}\&ap;-\frac{84.76\mathrm{ng}\&CenterDot;100\&CenterDot;256}{2\mathrm{GHz}\&CenterDot;256\mathrm{ms}}=-4.238\&times;{10}^{-12}g$

The result that heat transfer agent processing module 10 calculates is by externally equipment demonstration of output module 11.

Claims (3)

1. little quality testing sensing circuit based on FBAR comprises:

Be used to detect little mass loading as a FBAR of sensing sources with as the 2nd FBAR with reference to the source;

The one FBAR output signal is carried out first Fractional-N frequency circuit of frequency conversion and the second Fractional-N frequency circuit that the 2nd FBAR output signal is carried out frequency conversion;

Produce the timer of periodic counting signal based on the clock source;

Trigger first counter that the first Fractional-N frequency circuit output signal is counted and second counter that the second Fractional-N frequency circuit output signal is counted by count signal;

Receive first counter and the second rolling counters forward result, calculate the subtracter of both differences;

Receive subtracter result of calculation and calculate the heat transfer agent processing module of the quality of little mass loading;

Drive the clock source of first counter, second counter, timer and heat transfer agent processing module synchronous working.

2. little quality testing sensing circuit according to claim 1 is characterized in that: the frequency dividing ratio of described first Fractional-N frequency and the second Fractional-N frequency circuit is 2 ⁶～2 ¹⁰

3. little quality determining method based on FBAR may further comprise the steps:

A, provide, little mass loading is adsorbed on the FBAR as a FBAR of sensing sources with as the 2nd FBAR with reference to the source;

B, respectively the output signal of a FBAR and the 2nd FBAR is carried out frequency conversion;

C, in the fixed cycle, respectively the output signal of FBAR after the frequency conversion and the 2nd FBAR is counted, and is calculated both differences;

D, through type (I) calculate the quality of little mass loading;

$\mathrm{\&Delta;m}\&ap;-\frac{M\&CenterDot;S\&CenterDot;N}{f_s\&CenterDot;T}---\left(I\right)$

Wherein, M is the quality in FBAR piezoelectric working district; S is the difference in the step (c); f _sBe the original resonance frequency of FBAR; N is the frequency dividing ratio of frequency conversion in the step (b); T is the time of fixed cycle in the step (c); Δ m is the quality of little mass loading.

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